ScienceDirect. Performance Analysis in Off-Design Condition of Gas Turbine Air-Bottoming Combined System

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1 Available online at ScienceDirect Energy Procedia 45 ( 2014 ) th Conference of the Italian Thermal Machines Engineering Association, ATI2013 Performance Analysis in Off-Design Condition of Gas Turbine Air-Bottoming Combined System Carlo Carcasci*, Federico Costanzi, Beniamino Pacifici DIEF: Department of Industrial Enigineering University of Florence Via Santa Marta, 3 Florence (I) Abstract Nowadays, the gradual depletion of fossil fuels associated with constraints on emissions of greenhouse gases leads to valorize their wasted heat from power plant. One of the technologies adopted for improvement is the utilization of combined cycles. For this purpose, the steam cycle is used most frequently. These systems are highly efficient, but they are very complex and water is requested, moreover they are very heavy, so they cannot always be used. In this context, Air Bottoming Cycles (ABC) become attractive for potential use in future plants and repowering because they are light, compact and they have flexible-use and no water consumption. An application of an Air Bottoming Cycle (ABC) is composed of a gas turbine powered by natural gas, an air compressor and an air turbine coupled to the system by means of a heat exchanger, referred to as the AHX (Air Heat Exchanger). The aim of this paper is to study an Air Bottoming Cycle (ABC) that uses a medium power industrial gas turbine as topper cycle. A thermodynamic optimization is realized, determining the best pressure ratio and air mass flow rate of bottomer cycle. Then, an off-design analysis varying ambient temperature and FAR (Fuel Air Ratio) is shown, in fact, in this case, the exhaust gas conditions from topper gas turbine and inlet air of bottoming joule cycle change The The Authors. Authors. Published Published by Elsevier by Elsevier Ltd. Ltd. Open access under CC BY-NC-ND license. Selection and and peer-review peer-review under under responsibility responsibility of ATI of NAZIONALE ATI NAZIONALE. Keywords: Gas Turbine; Air Bottoming Cycle; Joule-Joule Combined Cycle; Off-design analysis; Ambient temperature variation; Fuel Air Ratio regolation. 1. Introduction Nowadays, the gradual depletion of fossil fuels associated with constraints on emissions of greenhouse gases leads to increasingly restrictive standards for heavy industries in terms of energy consumption and ambient impact. This * Corresponding author. Tel.: ; fax: address: carlo.carcasci@unifi.it The Authors. Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Selection and peer-review under responsibility of ATI NAZIONALE doi: /j.egypro

2 1038 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) fact pushes the large energy consumers to search for innovative solutions to valorize their wasted heat which could be a promising resource for the future. In this context, Air Bottoming Cycles (ABC) become attractive for potential use in future plants and repowering. An Air Bottoming Cycle (ABC) was proposed in the late 1980s as an alternative for the conventional steam bottoming cycle, which use hot combustion gases as a heat source for the air cycle. Some researchers call it "Joule- Joule Combined Cycle" (JJCC), others "Brayton Bottoming Cycle" (BBC). Air bottoming cycles are composed of a gas turbine powered by natural gas, an air compressor and an air turbine coupled to the system by means of a heat exchanger, referred to as the AHX (Air Heat Exchanger). The gas turbine engine has many advantages such as low investment costs, use-flexibility, low emissions, no-water consumption and short construction lead time. However, conventional industrial engines have low efficiencies. One of the technologies adopted nowadays for improvement it is the utilization of combined cycles. For this purpose, the steam cycle is used most frequently. These systems are highly efficient in terms of energy, but they are very complex and have high water consumption. An alternative to steam cycles are gas-air systems. So far gas-air systems can find application [1] in: The food industry (industrial bakeries, powdered milk factories); As a source of hot air in furnaces for glass melting; In systems of high-temperature furnaces where preheated air comes from the ABC; In offshore industry; gas-air systems can be used to improve the efficiency of simple power units with gas turbines operating at locations without access to large amounts of water. A performance analysis of this combined cycle was carried out by several authors [1 10]. In 1985, Farrel studied an air cycle thermodynamic conversion system which has been patented in 1988 [2]. Then, Wicks and Wagner [5] have studied a combined cycle with no steam nor cooling water requirements. The efficiency of an ABC cycle can be further increased by intercooling the air in the compressor stages as has been proposed by Najjar et al. [7] in In a recent study in 2010, Datta et al. [10] provided both energy and exergy analyses of an externally fired gas turbine cycle with an integrated biomass gasifier. They also investigated the effects of operating parameters like the pressure ratio and Turbine Inlet Temperature (TIT). Bhargava et al. [11] investigated some bottoming cycles for recovering waste heat energy available from the small/medium power turbines and they studied an Air Bottoming Cycle (called "Brayton Bottoming Cycle"). Furthermore, Facchini and Carcasci [12-17] have studied GT power plants in design and off-design conditions with a tool developed by themselves (called ESMS code), which has been applied in some thermodynamic cycles. The aim of this paper is to model an ABC cycle to calculate the combined cycle performance which uses a medium power industrial gas turbine as topper cycle. So, a thermodynamic analysis to optimize the bottomer cycle is realized; this system will be studied in off-design conditions varying the ambient temperature and using a FAR (Fuel-Air-Ratio) like a part-load control. Nomenclature m mass flow [kg/s] N Shaft speed [rpm] p Pressure [Pa] Q Heat [kw] s Specific entropy [kj/kgk] T Temperature [K] W Power [kw] Pressure ratio [-] Efficiency [-] Subscript amb bot Ambient Bottomer cycle

3 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) c des exh max pol PP Rec t top st Compressor Design/Nominal Exhaust Maximum Polytropic Pinch point Recovered Turbine/expander Topper cycle Stack Acronyms ABC AHX CC FAR GT TIT Air Bottoming Cycle Air Heat Exchanger Combined cycle Fuel Air Ratio Gas Turbine Turbine inlet temperature 2. The ESMS Cycle Analysis Code Power plants based on gas turbine engines are not very complex, but, to simulate them, a flexible and detailed tool is necessary. Gas turbine designers use ad-hoc code to simulate each component because a lot of details are necessary. The reader is referred to references [12-17] for a complete presentation of the code, related theory and some engineering applications. The most important feature of this modular simulation code is the ability to simulate a new power plant configuration without creating a new source program. The power plant configuration is defined by a connecting a number of elementary components representing different unit operations such as compressors, combustion chambers, mixers and so on. Each component is defined as a black box capable of simulating a given chemical and thermodynamic transformation. The resulting set of non-linear equations defining the power plant is then linearized (the coefficients are, however, updated in the course of the calculation). All equations are then solved simultaneously using a classic matrix method; thus the procedure is essentially that of the fully implicit linear approach. Simulation of design and off-design conditions consists of a two-step procedure. Adding heat transfer and momentum equations, by using the thermodynamic analysis, it is possible to realize the design of each component, so the main geometrical parameters will be determined (e.g. the velocity triangle at mean radius and other cascade parameters for the compressor or turbine, heat exchanger surface areas, etc.). Off-design performance simulation requires a geometric description of the different components determined in design-mode. These data result from a design study. When identifying the different parameters describing the component geometry, knowledge of some plant data is important to improve simulation results (e.g. the turbine exhaust flow rate and the temperature). Off-design simulations are based on fixed geometry (obtained in the course of the design study), and this results in a reduction in the number of input data. 3. Description of system and Topper GT 3.1. Power Plant description Figure 1 shows a typical model of air bottoming cycle. The topper cycle is a classical industrial cooled gas turbine, the bottomer one is a Joule cycle composed by a compressor and an air turbine/expander, coupled by means of an Air Heat Exchanger (AHX). The main advantage of the system is its simplicity. The mechanical power obtained

4 1040 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) GE10 data sheet Value Unit ISO Rated Power kw Heat Rate kj/kwh Electrical Efficiency Pressure Ratio Exhaust Flow Turbine Speed Exhaust Temperature kg/s rpm C Table 1. GE10 turbine data sheet [18,19]. Fig. 1. Air Bottoming Cycle scheme ESMS results GE cataloque [18,19] 50 m exh [kg/s] ESMS results GE data [18,19] W [kw] T amb [ C] Fig. 2. Exhaust flow GE data compared with simulation data T amb [ C] Fig. 3. Output power GE data compared with simulation data. from the turbine may be used either to support the gas turbine system or to generate electricity. Due to the short start-up time of the air turbine, the ability to meet the peak demand for power may also be significant. There is no water consumption and no combustion process and there are no toxic emissions in the bottomer cycle. The crucial element is the AHX structure, which has a decisive impact on the efficiency of the entire system. A rationally designed system must take account of the differences in the medium temperature which determine its size, as well as the pressure drops which determine the efficiency of both the air and the gas turbine. A high efficiency of the system is obtained for small temperature differences in the heat exchanger Topper Gas Turbine Usually ABC system is not suitable for great size plants, so it will be analyzed a middle size industrial gas turbine. In this study, the performances of an ABC cycle based on the commercially available GE10 engine are examined. The topper cycle, used in this work, is a real gas turbine (GE10, [18, 19]), normally applied by GE in industrial process; there are many units running under conditions ranging from the cold of Alaska to the heat of the desert. Its efficiency and operational flexibility make the GE10 a cost-effective choice for all applications. The engine is of the single-shaft type. The compressor is an eleven-stage axial flow design with a 15.5:1 pressure ratio derived from GE Aircraft Engine transonic flow aero design technology with inlet guide vanes. The rotational speed is rpm with a mass flow of 47.5 kg/s. The turbine consists of three reaction stages. In the first two-stages, the hot gas parts are cooled by air extracted from the axial compressor. The GE10 is available in both diffusion combustion system and DLN (Dry Low NOx) versions and it is able to burn a wide range of liquid and gas fuels, including Low BTU gas and hydrogen. Gas turbine GE10, whose basic data are listed in Table 1, is used to model the topper; they are referred to ISO

5 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) conditions of GE10 working. In the first step, the design simulation of GE10 at 15 C, sea level, imposing the data in table 1 was done. For this simulation, using a few known operating characteristics (compressor ratio, turbine exhaust temperature and mass flow rate, gross efficiency, output power, number of compressor and turbine stages), some general design parameters can be determined. GE10 simulation has very good agreement with the manufactures performance data. The off-design performance, varying ambient temperature is compared with the catalogue curve (Figure 2 and 3). Good agreement is achieved in the right-hand zone, using a constant turbine inlet temperature. The results of present simulation are obtained without changing the IGV (Inlet Guide Vane), thus, probably, the disagreement in the lefthand zone is due to this hypothesis. 4. Design of Bottomer Cycle In this paper, more attention is put on thermodynamic and performance analysis of the bottomer cycle. In this analysis, the polytropic efficiencies of bottomer compressor and the expander are imposed ( c,bot,pol =0.90; ( t,bot,pol =0.885), in addition to pressure losses ( p=4%) and pinch point temperature difference ( T PP =35 C, considering that the heat is exchanged between two gas) in the Air Heat Exchanger (AHX). The topper GE10 gas turbine has been simulated in off-design configuration, leaving the ISO condition but considering the output pressure loss, thus the exhaust pressure is higher and the output power decreases; while the bottoming cycle is simulated using the design analysis. Now, varying the pressure ratio and the air mass flow in the bottomer cycle, the output power and efficiency are optimized. In the first part of this work, the best air mass flow and pressure ratio of the bottomer cycle are searched. Initially, to determine the bottomer cycle best air mass flow, it is necessary to fix a pressure ratio for the bottomer compressor. Figure 4 shows the output power of bottoming cycle. Fixing the pressure ratio, the curve presents a discontinuity on first derivative, so a peak is present. The output power depends on air mass flow rate and specific work. When the air mass flow rate is little, in the AHX, the air temperature variation is greater than hot gas temperature variation, thus pinch point temperature different is imposed in the "hot side" of AHX and so the inlet temperature of expander is fixed. The specific work depends on pressure ratio and inlet temperature, so it is constant. Thus, in the left side of peak, the output power grown is due only to air mass flow rate. The stack temperature of hot gas decreases when the air mass flow rate grown (Figure 5), it means that the recovered heat is not the maximum. On the other hand (on the right side of peak), the air temperature variation in the AHX is less than gas temperature variation, so the pinch point temperature difference position is on the "cold side" of AHX and the recovered heat is the maximum, but the expander inlet temperature decreases, so the efficiency of Brayton bottoming cycle decreases. When the temperature difference between the hot gas and air are equal to pinch point temperature difference ( T PP ) both in the "hot side" and "cold side" of AHX, the maximum recovery heat and thermodynamic efficiency of bottoming cycle are the maximum. Bhargava et al. [11] obtained similar trends. The T pp on hot side T [K] Hot gas stack Expander inlet T pp on cold side Fig. 4. Output power of bottoming cycle varying air mass flow for two different pressure ratios m air,bot [kg/s] Fig. 5. Air and hot gas temperature surrounding the AHX.

6 1042 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) W cc [kw] cc [-] bot [-] Fig. 6. Trend of combined cycle power versus pressure ratio bot [-] Fig. 7. Trend of combined cycle efficiency versus pressure ratio. Topper Bottomer p= Pa T= K = 15.5 CC Fuel W=11250 kw N=11000 rpm =0.314 T [K] St top m=47.5 kg/s p= Pa T=443 K C top T top Cool Air Gas AHX T pp =35K T pp =35K m=47.5 kg/s p= Pa T= K W=2510 kw N=7900 rpm =0.17 = 3.5 C bot T bot s[ kj/kgk] Fig. 8. T-s diagram of bottomer and topper cycles in design conditions. m=49.3 kg/s p= Pa T= K Air m=49.3 kg/s p= Pa T=428 K m=49.3 kg/s p= Pa T= K m=49.3 kg/s p= Pa T=540 K Fig. 9. Air bottoming cycle scheme with main thermodynamic data. Air St bot best point for bottomer cycle power is found for an air mass flow value (m bot =49.3 kg/s; m bot /m top,exh =1.038) that is valid for all pressure ratio, as figure 4 shows. In the best point, the ratio between air and hot gas mass flow rate is equal to specific heat ratio between hot gas and air. For this reason, mass flow rate ratio is about 1. In the next step, fixing the air mass flow, the bottomer compressor pressure ratio is varied to find the maximum output power (Figure 6). Increasing the bottoming pressure ratio, the inlet air temperature into AHX increases, so the heat recovered by hot gas decreases (the stack hot gas temperature increases), but thermodynamic efficiency of Brayton bottoming cycle increases, so the curve presents a maximum value. The trend of combined cycle power and thermodynamic efficiency is displayed in figure 6 and 7, where the best pressure ratio ( bot 3.5) of bottoming cycle is shown. As results of this analysis, combined cycle power of 13,76 MW and a thermal efficiency of =0.39 have been found. The thermodynamic efficiency is increased about 7.6 percentage point relative to the basic GE10 turbine. In figure 8 is shown the Joule cycles representing the design configuration in a T-s diagram, where the recovered heat can be observed. Moreover, the temperature differences between hot gas and air can be visualized. In figure 9, the power plant scheme is shown with main parameter values in each point. The determination of the size of the outlays is based on the information obtained by Chmielniak et al. [1]. The purchase cost of individual gas-air system components is strongly dependent on the mass flow, as well as on the internal efficiency of the compressor and of the turbine. The lower price of the compressor and the turbine for an ABC system compared to the machinery operating in a gas turbine system results mainly from the lower pressure

7 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) Fig.10. Output power varying ambient temperature. Fig. 11. Topper and bottomer inlet mass flow varying ambient temper. Fig. 12. Heat recovered in the AHX varying ambient temperature. Fig. 13. Pressure ratios varying ambient temperature. and temperature values in the cycle. The purchase cost of GT-ABC is smaller in comparison to a classical steam combined system installation, in fact there is a saving of about 25% on the unit investment expenditure. The use of this type of installation may take place if the economic analysis is strongly affected by the main advantages of GT-ABC systems, such as the low water consumption, operation flexibility, the potential to meet the peak demand for energy and the light weight. 5. Off-design analysis 5.1. Ambient temperature analysis The second part of this work is dedicated to an off-design analysis. Before, the ambient temperature variation is studied. Obviously, it varies for both Brayton cycles at the same time; so varying the ambient temperature, the topper cycle changes its performance and changes the exhaust gas flow parameters which influence the bottomer cycle and it is influenced from ambient temperature variation, too. In this analysis, topper gas turbine and bottomer cycle are simulated using the off-design analysis. The design temperature point is fixed to 15 C, the range in off-design simulation is from -25 C to 45 C. In figure 10, the bottomer cycle power decreases in a little range while topper and combined cycles trends are steeper; however, varying ambient temperature, the output ratio between bottoming cycle and topper cycle is about

8 1044 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) Fig. 14. Exhaust temperatures varying ambient temperature. Fig. 15. Pinch point temp. differ. of AHX varying ambient temperature. Fig. 16. Output power varying TIT. Fig. 17. Thermodynamic efficiency varying TIT. constant. These trends are due to the inlet air mass flow reduction (Figure 11), in fact the heat recovered decreases, increasing the ambient temperature (Figure 12). Also specific work decreases because the pressure ratios, both in the topper and bottomer cycle, decrease (Figure 13) because the compressor inlet temperature grows. However, decreasing the pressure ratio, the exhaust temperatures increase (Figure 14) and the higher exhaust temperature of topper gas turbine determines an advantage on specific work of bottomer cycle. In figure 15, the pinch point temperature different trend is shown Fuel-Air Ratio variation analysis In a gas turbine, for part-load condition, the FAR (Fuel Air Ratio) control system is often used, so the turbine inlet temperature decreases. In this part, an off-design analysis varying the maximum temperature of the topper gas turbine is done. The range of topper maximum temperature (TIT) is from -300 C to +50 C respect to design condition. The bottomer cycle power increases in a little range while topper and combined cycles trends are steeper (Figure 16) and so thermal efficiencies have the same increasing trend varying the maximum temperature (Figure 17). The disadvantage in term of thermal efficiency is more relevant in the bottomer cycle (this one has a repercussions on

9 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) Fig. 18. Main exhaust temperatures varying TIT. Fig. 19. Exhaust mass flow rates varying TIT. Fig. 20. Topper and bottomer pressure ratio varying TIT Fig. 21. Heat recovered in the heat exchanger AHX varying TIT. combined cycle efficiency) because it is affected by the decreases of specific work. In fact, the exhaust temperature from topper turbine decreases (Figure 18) when the maximum temperature is lower, so the inlet expander temperature of bottoming cycle (Figure 18) decreases, too. The mass flow rate change slightly (Figure 19). The pressure ratio (Figure 20) and the heat recovered (Figure 21) are lower when the FAR is activated, in fact, decreasing the exhaust temperature from gas turbine, the maximum temperature of bottoming cycle is lower. The heat recovered by heat exchanger AHX (Figure 21) conveys of the pressure ratio drop (Figure 20) because the inlet air temperature into heat exchanger depends on it. Conclusions The analysis presented in this paper shows that the considered gas turbine system coupled with an air bottoming cycle (ABC) is very interesting from the point of view of power engineering. The power and the thermal efficiency are higher than that of standalone gas turbine units. A bottomer cycle design analysis is realized and +7.6 percentage point on thermal efficiency and +22.3% on output power respect to the basic GE10 turbine are determined. The air mass flow rate of bottoming cycle is slightly higher than topper gas turbine, but the pressure ratio is low (about bot =3.5) and it depends only from exhaust gas turbine temperature. Then, an off-design model of the topper gas turbine and bottomer cycle are used to determine the performance of

10 1046 Carlo Carcasci et al. / Energy Procedia 45 ( 2014 ) combined cycle varying ambient temperature and in part load condition. Increasing the ambient temperature, as well known, the output power of topper gas turbine decreases, and the bottomer cycle decays too, because the pressure ratio and mass flow rate are lower. The bottomer cycle is affected less because increasing the ambient temperature, the exhaust temperature from topper gas turbine is higher. Using the Fuel Air Ratio control system, the exhaust temperature of topper gas turbine decreases and this determines a worse performance of bottoming cycle. In fact, the recovered heat decays decisively. Acknowledgements The authors are grateful to Prof. Bruno Facchini for his support and his supervision. References [1] T. Chmielniak, D. Czaja, S. Lepszy. "Technical and Economic Analysis of the Gas Turbine Air Bottoming Cycle". ASME Turbo Expo 2012, June 11-15, 2012, Copenhagen, Denmark, GT [2] Farrel WM. "Air cycle Thermodynamic Conversion System". US patent 4,751,814, [3] M. Ghazikhani, M. Passandideh-Fard, M. Mousavi. "Two New High-Performance Cycles for Gas Turbine with Air Bottoming". Elsevier, Energy vol.36, 2011, pp [4] M. Korobitsyn. "Industrial applications of the air bottoming cycle". Pergamon, Energy Conversion and Managem., vol.43, 2002, pp [5] Wicks F, Wagner C. Synthesis and evaluation of a combined cycle with no steam nor cooling water requirements. Proceedings of 28th Intersociety Energy Conversion Engineering Conference IECEC 93, vol. 2, Boston, 1993, p [6] Bolland O., Forde M., Hande B. Use of Gas Turbine Waste Heat for Power Generaton. J. of Engineering for Gas Turbines and Power, 1996, Vol.118, Issue2, 359. [7] Najjar Y. S. H., Zaamout M. S. Performance Analysis of Gas Turbine Air-Bottoming Combined System. Energy Conversion And Management Vol. 37, No. 4, pp , [8] Weston KC. "Dual gas turbine combined cycles". Proceedings of 28th Intersociety Energy Conversion Engineering Conference IECEC 93, vol.1, Boston, p [9] Wagener MTPA. The Dual Gas Turbine Combined Cycle. Feasibility Study of a New Combined Heat and Power Concept. MSc Thesis, University of Twente Stork Comprimo, The Netherlands, August, [10] Datta AM, Ganguly R, Sarkar L., "Energy and Exergy Analyses of an Externally Fired Gas Turbine (EFGT) Cycle Integrated with Biomass Gasifier for Distributed Power Generation". Energy, 2010; 35, [11] Bhargava, R.K., Bianchi, M., De Pasquale, A., 2011; "Gas Turbine Bottoming Cycles for Cogenerative Applications: Comparison of Different Heat Recovery Cycle Solutions"; Proc. of ASME Turbo Expo 2011, June 6-10, 2011, Vancouver, Canada, GT , doi: /gt [12] Carcasci C., Facchini, B., 1996; "A Numerical Method for Power Plant Simulations". ASME J. of Energy Resources Technology, March 1996, 118. [13] Carcasci C., Facchini B., Marra R.; 1996, "Modular Approach to Off-design Gas Turbines Simulation: New Prospect for Reheat Applications". IGTI ASME Turbo Expo '96, Paper 96-GT-395, Birmingham U.K., June [14] Carcasci C., Facchini B., Harvey S., 1998; "Design and Off-Design Analysis of a CRGT Cycle Based on the LM2500-STIG Gas Turbine", ASME International Gas Turbine and Aeroengine Congress & Exhibition, Paper 98-GT-36, Stockolm (S), June [15] Carcasci C., Facchini B., Harvey S. "Design Issues and Performance of a Chemically Recuperated Aeroderivative Gas Turbine." Proc. Instn Mech Engrs, Part A, Journal of Power and Energy, 1998, 212, A(04398), [16] C. Carcasci, B. Facchini, S. Gori, L. Bozzi, S. Traverso. Heavy Duty Gas Turbine Simulation: Global Performances Estimation And Secondary Air System Modifications. ASME Turbo Expo 2006 Power for Land, Sea & Air May 8-11, 2006, Barcelona, Spain GT [17] Carcasci C., Da Soghe R., Silingardi A., Astrua P., Traverso S. Heavy Duty Gas Turbine Simulation: a Compressor IGV Airfoil Off-Design Characterization. ASME TURBO EXPO 2010: Power for Land, Sea & Air, GT2010, June 14-18, 2010, Glasgow, UK, GT [18] GE data sheet, [19] GE Oil&Gas, "GE 10-1 Gas Turbine", brochure COMK/MARK 839/II - Printed by: Sagraf , 2005 Nuovo Pignone S.p.A.